Polyhydroxyalkanoates as Packaging Materials: Current Applications and Future Prospects - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 188

as opposed to abiotic weathering. In order to be considered and marketed as
biodegradable, plastics must satisfy ISO standards as per Table 8.2. 31

8.4 Polyhydroxyalkanoates (PHAs)
Polyhydroxyalkanoates (PHAs) are a family of biopolyesters synthesised by a
diverse range of microorganisms under conditions of excess carbon and one
or more limiting nutrients. 32 Under these unbalanced conditions, PHAs are
stored as discrete intracellular inclusion bodies where they serve as an en-
ergy storage system in a similar manner to glycogen in mammals; in certain
genera they may also serve as an ion sink and play an important role in
encystment and sporulation. 33,34 Microorganisms subsequently catabolise
PHAs into carbon and hydrogen when needed. Flexibility in the bioproces-
sing systems, in particular the carbon source, leads to different monomeric
compositions; at the last count, there were over approximately 150 different
monomeric components. 35 By far the greatest group of PHAs occur with
variations within the side chain of the monomer (Figure 8.1(a)). Although,
more recently there have been a number of PHAs possessing variations in
the back bone length. 35
PHAs are classified according to the number of monomeric carbons, those
of relatively short chain length (sclPHAs) with three to six carbons, those
with seven to 16 carbons as mclPHAs (medium chain length) and PHAs with
monomers possessing more than 16 carbons as long chain length, lclPHAs
(Figure 8.1). In addition to simple variations in the length of the chain
backbone and side chain, a number of PHAs containing functional chemical
groups have also been synthesised. These include halogens, branched and
aromatic as well as unsaturated groups. 35 The most common PHA packaging
resins are polyhydroxybutyrate (PHB) and its copolymer with poly-
hydroxyvalerate (PHB-co-HV), (Figure 8.1(b), (c)).

d n 2 r 4 n g | 3

.

Figure 8.1 Chemical formulae for the family of polyhydroxyalkanoates (a) and the
two most common commercial plastic resins, polyhydroxybutyrate (b)
and its copolymer with polyhydroxyvalerate (c).